add CombinationSum and test (#6725)

* add CombinationSum and test

* Format array and list literals in CombinationSumTest

* Fix sorting comparator in CombinationSumTest

* Refactor CombinationSum for better handling and clarity

Updated combinationSum method to handle null or empty candidates and improved variable naming for clarity.

* Fix sorting comparator in CombinationSumTest

* Update CombinationSumTest.java

* Fix sorting comparator for list of integers

* Fix formatting issues in CombinationSum class

* Change CombinationSum class to final

* Refactor norm method to accept Iterable instead of List

* Remove unnecessary whitespace in CombinationSumTest

src/test/java/com/thealgorithms/backtracking/CombinationSumTest.java

@@ -0,0 +1,29 @@
+package com.thealgorithms.backtracking;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class CombinationSumTest {
+    private static List<List<Integer>> norm(Iterable<List<Integer>> x) {
+        List<List<Integer>> y = new ArrayList<>();
+        for (var p : x) {
+            var q = new ArrayList<>(p);
+            q.sort(Integer::compare);
+            y.add(q);
+        }
+        y.sort(Comparator.<List<Integer>>comparingInt(List::size).thenComparing(Object::toString));
+        return y;
+    }
+
+    @Test
+    void sample() {
+        int[] candidates = {2, 3, 6, 7};
+        int target = 7;
+        var expected = List.of(List.of(2, 2, 3), List.of(7));
+        assertEquals(norm(expected), norm(CombinationSum.combinationSum(candidates, target)));
+    }
+}